Macrostomion gnathothlibi spec. nov. is described from a series reared gregariously from a larva of the sphingid moth Gnathothlibus eratus eras (Boisduval) collected in Papua New Guinea. Dedanima Cameron, 1903, is synonymised with Macrostomion Szépligeti, 1900. A further four gregarious broods of Macrostomion are noted, three certainly and one probably reared from mummified larvae or prepupae of Sphingidae, from which it appears that Macrostomion species may all be gregarious parasitoids of Sphingidae.